There are a variety of applications in which it is desirable to maintain a degree of humidity control. One category of these applications involves devices with electronic circuitry provided within some form of closed housing. The humidity within the housing needs to be controlled in order to limit or avoid corrosion of metal within the circuitry, or condensation of moisture on exposed radio frequency (RF) circuit lines. As one specific example of this type of application, there is a need to control humidity in electronic assemblies which contain RF gallium arsenide (GaAs) monolithic microwave integrated circuit (MMIC) devices, in order to enhance the overall reliability of the MMIC devices.
Phased array antenna systems often involve the use of RF MMIC devices. In such an antenna array, which is often referred to in the art as an active electronically scanned array (AESA), the spacing between antenna elements is determined by the operational frequency of the system. Consequently, as the operational frequency increases, the inter-element spacing between adjacent antenna elements decreases, which in turn decreases the size of the array and the amount of space available for the circuitry associated with each antenna element. In a traditional low-frequency antenna array, the array is relatively large, and there is a fair amount of space available in association with each antenna element. Due in part to the amount of available space, each MMIC is typically disposed within its own separate hermetic package. However, as the operating frequency increases, the use of a separate hermetic package for separate components becomes difficult or impossible. For example, in an active electronically scanned array (AESA) antenna operating in the Ka frequency band at approximately 35 GHz, the spacing between adjacent antenna elements will typically be about one-half of a wavelength, or in other words only about 0.175 inch.
One way in which moisture can enter an electronic package is to leak past a moisture seal, such as an O-ring. An entirely different way in which moisture can enter such a package is through permeation. Permeation refers to the passage of a molecule through a solid barrier, and is different from leakage. In particular, permeation occurs when gas molecules dissolve into one side of a material such as a housing wall, diffuse through that material, and then evaporate from the far side of the material. In an AESA having a housing which includes a radome, the radome is often made from a material which is more susceptible to moisture permeation that other portions of the housing.